Mechanical heart valve

Details Mechanical heart valve Mechanical heart valve

1999-06-01

2002-05-28

Robert, Eduardo C. (Department: 3732)

Prosthesis (i.e., artificial body members), parts thereof, or ai

Heart valve

Having rigid or semirigid pivoting occluder

Reexamination Certificate

active

06395024

ABSTRACT:

BACKGROUND OF THE INVENTION
FIELD OF INVENTION
The present invention relates to an improved trileaflet mechanical heart valve. More specifically, the present invention relates to a trileaflet mechanical heart valve with improved flow characteristics. Such a mechanical heart valve is useful for surgical implantation into a patient as a replacement for a damaged or diseased heart valve.
BACKGROUND CONSIDERATIONS
There are numerous considerations in the design and manufacture of a mechanical prosthetic heart valve. An important consideration is the biocompatibility of the materials used in the prosthesis. The materials used must be compatible with the body and the blood. Furthermore, the materials must be inert with respect to natural coagulation processes of the blood, i.e., they must not induce thrombosis (an aggregation of blood factors, primarily platelets and fibrin with entrapment of cellular elements, frequently causing vascular obstruction at the point of its formation) when contacted by the blood flow. A local thrombus can give rise to an embolism (the sudden blocking of a blood carrying vessel) and can even under certain circumstances hinder proper valve operation. Numerous materials have been tested for such desirable biocompatibility. Several materials are commonly used for making commercially available prosthetic heart valves (materials such as stainless steel, chromium alloys, titanium and its alloys, and pyrolytic carbon).
Another consideration in the design and manufacture of a mechanical prosthetic heart valve is the valve's ability to provide optimum fluid flow performance. Mechanical prosthetic heart valves often create zones of turbulent flow, eddies, and zones of stagnation. All of these phenomena can also give rise to thrombosis and thrombo-embolisms. Biological valves (or bioprostheses) emulate the form and the flow pattern of the natural heart valve and thus have better fluid flow performance over conventional mechanical prostheses. Such bioprosthetic valves do not require long-term anti-coagulant medication to be taken by the patient after implantation at least in the aortic position. These two thrombus-generating factors (materials used and flow characteristics) are problematic in conventional mechanical heart valve prostheses. Thus, patients who currently receive a mechanical heart valve prosthesis require a continuous regime of anti-coagulant drugs which can result in bleeding problems. The use of anti-coagulant drugs therefore constitutes a major drawback of mechanical heart valve prostheses when compared with bioprostheses.
However, biological replacement valves suffer from problems too. As clinical experience has indicated, unlike mechanical valves, their life-span of is often too short. Because of the progressive deterioration of bioprostheses, they often need to be replaced via costly additional major surgery.
Yet another consideration in the design and manufacture of a mechanical prosthetic heart valve concerns the head loss (pressure drop) associated with the valve. This head loss occurs during the systolic ejection or diastolic filling of a ventricle. In conventional designs, some head loss is inevitable since it is inherent to the reduction in the effective orifice area of the mechanical prosthetic heart valve as compared to natural valves. The reduction in effective orifice is caused by the sewing ring which is conventionally required for surgical installation of the prosthetic valve, by the thickness of the valve housing, and by the hinges which enable the valve's flaps (leaflets) to move between an open and closed position. Another portion of the head loss is due to the geometric disposition of the valve's flaps with respect to the flow of blood.
As mentioned above with respect to the progressive deterioration of bioprostheses, durability is another consideration in the design and manufacture of a mechanical prosthetic heart valve. A mechanical prosthetic heart valve should demonstrate a mechanical lifetime equivalent to approximately 380-600 million cycles (i.e., the equivalent of about 15 years). Obviously, the mechanical lifetime is related to the geometrical design of the valve as well as the mechanical characteristics of the materials used.
Of course, the valve's ability to minimize leakage is also important. Leakage generally comprises regurgitation (backward flow of blood through the valve during operation, and otherwise known as dynamic leakage) and static leakage (any flow through the valve in the fully closed position). In the conventional valves, the amount of regurgitation is at least 5% of the volume of blood flow during each cycle, and is often more. When a patient has two prosthetic valves on the same ventricle, regurgitation (dynamic leakage) thus comprises at least about 10% (leakage on the order of several hundred L per day). Thus, dynamic leakage clearly puts undesirable stress on the heart muscle. Static leakage, on the other hand, is typically caused by the imperfect mechanical sealing of the prosthetic valve when its flaps are closed. Because static leakage also causes the heart muscle to work harder, it must be taken into consideration in the design and manufacture of a mechanical prosthetic heart valve.
The closing mechanism of natural cardiac valves has not been taken into account in the design of conventional mechanical valve prostheses. When the flow rate across the valve becomes zero, the natural aortic valve is already more than 90% closed. In contrast, conventional mechanical valve prostheses at that same time remain almost fully open. From this almost fully open position, conventional mechanical valve leaflets abruptly close with the large amount of regurgitation. In an aortic position, this occurs at the very beginning of the diastole, and in the mitral position, this occurs even more abruptly at the very beginning of the systole. In conventional mechanical leaflets, the mean closing velocity of some portions of the leaflets (at 70 beats per minute) is on the order of 1.2-1.5 m/sec, whereas the highest closing velocity in a natural valve is 0.60 m/sec. Rapid angular closing speeds create cavitation in mechanical prosthetic heart valves. This high closure speed increases the intensity of the impact of the leaflets upon closure and thus, generates sufficiently large acoustical vibrations to cause discomfort in the patient, damage the blood (embolisms), and generates micro-bubble formations in the blood which may be detected by a transcranial doppler (HITS—High Intensity Transcranial Signals).
Thus, conventional mechanical heart valves suffer from several disadvantages. First, conventional mechanical heart valves fail to provide optimal blood flow characteristics. Next, conventional mechanical heart valves allow blood to stagnate behind the valve leaflets, thus creating the possibility of blood clotting in those locations. Also, conventional mechanical heart valves may not provide optimum opening and closing times (e.g., times which properly emulate a natural human valve). It has not been possible, in the past, to reproduce the flow characteristics of a natural valve when using a mechanical prosthesis. Thus, with the use of conventional mechanical heart valves, embolic incidents and subsequent mortality may be directly or indirectly linked to the valve prosthesis.
Accordingly, there is a need for an improved mechanical heart valve for implantation into a patient which provides improved flow characteristics, minimizes blood clotting behind the leaflets, and provides more natural opening and closing behavior.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to an improved mechanical heart valve for surgical implantation into a patient which substantially eliminates one or more of the problems or disadvantages found in the prior art.
An object of the present invention is to provide for an improved mechanical heart valve for surgical implantation into a patient which provides improved flow characteristics.
Another object of the present invention is to provide for an improv

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